1. Technical Field
The present invention relates to a surface hardening method for a metal member of an engine or a driving member in the field of a transport or machine structure and a production method of the member, wherein the member requires abrasive resistance.
2. Background Art
Recently, with crude oil becoming heavy, demand for light oil has increased and oil refining methods have changed so that fuel characteristics of low quality fuel oil has worsened. As a result, parts of an internal combustion engine tend to be worn by hard particles, sulfur content, and fuel residue included in the fuel.
For dealing with the above situation, for example, as an art for improving abrasive resistance of a ring groove of an integral FCD (Ferrum Casting Ductile) piston used mainly in a diesel engine, there is a well-known and widely adopted laser quenching art wherein that the ring groove is quenched by laser (for example, see the Patent Literature 1), a high frequency quenching art (for example, see the Patent Literature 2) and a chromium plating art.    Patent Literature 1: the Japanese Patent Laid Open Gazette Sho. 61-149424    Patent Literature 2: the Japanese Patent Laid Open Gazette Hei. 7-119831
However, the hardness of the quenched part treated by the laser quenching art or the high frequency quenching art is about 600 to 800 Hv. The structure of the quenched part is not perfectly uniform and structures such as martensite, bainite and retained austenite are included, whereby the hardness is not uniform.
A top ring groove is susceptible to engine combustion temperature and when the ring groove is heated above about 150° C., the quenched structure is tempered and the hardness is reduced for about 100 to 200 Hv.
Furthermore, when the piston is made from cast iron which is corroded easily, the ring groove is corroded by sulfur or the like included in fuel gas.
Because of the above reasons, especially in an internal combustion engine, when the piston ring groove is worn by use for hours and a vertical gap is enlarged by abrasion, lubricating oil is raised excessively by pumping action so that consumption of lubricating oil is increased. When the abrasion reaches a fixed amount, the piston must be exchanged, whereby the maintenance requires cost.
The laser quenching causes many shattered cracks when the quenching depth is larger than about 300 μm deep quenching is unable to be performed. On the other hand, the high frequency quenching can perform quenching with depth not larger than about 800 μm, but causes large heat treatment distortion and requires post processing such as grinding.
With regard to surface hardening by chromium plating, the plating part is hard for about 800 to 1000 Hv and has high abrasive resistance, but the plating is very expensive. Furthermore, the plating solution contains hexavalent chromium and is not preferable in consideration of influence of this material upon the environment.